Gas appliances such as furnaces and hot water heaters require a connection between appliance and a vent pipe to conduct combustion products including noxious gasses and water vapor to the atmosphere.
The prior art connection includes straight sections of smooth wall pipe including elbows to make the required turns between the discharge opening in the gas appliance and the ceiling or wall vent. Because of the infinite varieties of distances between the gas appliance and the vent as well as the infinite positions with respect to the gas appliances and the vent, it has been the prior art practice for journeymen sheet metal workers to install elbows on the gas appliance and the vent and then to cut a straight length of pipe to fit between the elbows. The elbows and straight length of pipe are connected to one another by crimping one end of the pipe and elbows, forcing the parts together and securing them by at least three stainless steel screws at each joinder.
The process of selecting elbows with the correct angle and measuring and cutting the straight length of pipe plus crimping the ends of the elbows and straight section of pipe is tedious, time consuming and often unsightly.
In earthquake country, such connections are relatively hazardous since substantial relative movement between the gas appliance and the vent can cause the rigid connection to twist and bend which at the least will cause partial collapse of the pipes and at worst, actual separation of one or more of the joints.
With the advent of thermally efficient gas appliances a new problem has surfaced in the industry which supplies gas appliance connections; viz., there is a substantial increase in condensation occurring in the connector and serious corrosion problems have arisen. To make the corrosion problem even worse, the flue gasses from these high efficiency furnaces are loaded with chlorine ions which are hygroscopic in nature (affinity for water). The higher concentration of acid in the vent connector makes them particularly vulnerable to that type of corrosion that pits the metal thereby quickly forming holes through the metal. Condensation, if permitted to repeatedly stay in contact with the inner wall of the conduit for any appreciable amount of time will result in rusting and pitting of a metal connector.